1. Field of the Invention
The present invention relates to a liquid crystal display device and a method for producing the same. More specifically, the present invention relates to a liquid crystal display device having a display medium with a structure in which liquid crystal regions are partitioned by a polymeric material and a method for producing the same.
2. Description of the Related Art
As display devices utilizing an electro-optic effect, liquid crystal display devices using nematic liquid crystals have conventionally been used. Examples of such liquid crystal display devices include a twisted nematic (TN) liquid crystal display device and a super-twisted nematic (STN) liquid crystal display device. Liquid crystal display devices using ferroelectric liquid crystal have also been proposed. These liquid crystal display devices include a pair of glass substrates, nematic liquid crystal or smectic liquid crystal provided between the substrates, and two polarizing plates sandwiching the substrates.
Furthermore, as the display devices utilizing an electro-optic effect, liquid crystal display devices utilizing a light scattering phenomenon of liquid crystal, instead of using the polarizing plates, have been known. Such liquid crystal display devices use a dynamic scattering (DS) mode and a phase change (PC) mode.
In recent years, liquid crystal display devices requiring no alignment treatment have been proposed. Such a liquid crystal display device electrically regulates a transparent state and an opaque state by using the birefringence of a liquid crystal. More specifically, in such a liquid crystal display device, the refractive index of liquid crystal molecules with respect to ordinary light is matched with the refractive index of a supporting medium which supports the liquid crystal. Thus, liquid crystal molecules are oriented under the application of a voltage and hence a transparent state is displayed; whereas the orientation of the liquid crystal molecules is disturbed under the application of no voltage and hence a light scattering state is displayed. Japanese National Publication No. 61-502128 discloses a specific method: liquid crystal and photopolymerizable or thermosetting resin are mixed and resin is cured to deposit liquid crystal, whereby liquid crystal droplets are formed in the resin.
Japanese Laid-Open Patent Publication Nos. 4-338923 and 4-212928 disclose a liquid crystal display device using polarizing plates with improved viewing angle characteristics, i.e., a polymer dispersed liquid crystal device sandwiched with polarizing plates disposed so that the respective polarizing directions cross at right angles (hereinafter, referred to as crossed polarizing plates). These conventional liquid crystal display devices have improved viewing angle characteristics. However, the use of scattering of light for the elimination of polarization makes the brightness of the device 1/2 that of a TN liquid crystal display device. Thus, these conventional devices have not found a wide range of use.
Furthermore, Japanese Laid-Open Patent Publication No. 5-27242 discloses a method for improving viewing angle characteristics by disturbing the orientation of liquid crystal molecules with walls or projections of a polymer to form random domains. However, according to this method, liquid crystal domains are formed at random, a polymeric material enters a pixel portion, and a plurality of disclination lines formed at random between the liquid crystal domains are not eliminated even under the application of a voltage. For these reasons, the conventional liquid crystal display devices have the disadvantage of low contrast, light transmittance under the application of no voltage is low, that is, the black level is not satisfactory under the application of a voltage.
Accordingly, the conventional liquid crystal display devices using polarizing plates have poor viewing angle characteristics and hence are unsuitable for use as a liquid crystal display device for a wide viewing angle. For example, a TN liquid crystal display device undergoes alignment treatment so that liquid crystal molecules rise in the same direction under the application of a voltage. That is to say, the TN liquid crystal display device has a structure in which liquid crystal molecules have an initial orientation of 90.degree. twist and rise in one direction at a certain angle, i.e., a pretilt angle. This makes the liquid crystal molecules tilt in the same direction in the case where a gray scale display is conducted to allow the liquid crystal molecules to rise, as shown in FIGS. 51(a) to 51(c). Because of this, as shown in FIG. 51(b), when the liquid crystal molecules are viewed from directions A and B, apparent refractive indices become different. This also makes the difference in contrast between directions A and B large, and in some cases, results in an abnormal display such as a change in hue and the reversal of black and white colors.
As described above, the conventional liquid crystal display devices have the disadvantage of poor viewing angle characteristics.
Another method for producing a liquid crystal display device using polarizing plates has been proposed. According to this method, first, a mixture containing liquid crystal and a photopolymerizable material is provided between a pair of substrates. Then, light is irradiated to the mixture to a predetermined pattern through a photomask. At this time, the liquid crystal is phase-separated from the polymeric material in a regular manner. As shown in FIGS. 52(a) to 52(c), when a voltage is applied to the device thus produced, liquid crystal molecules interact with the polymer and consequently, the liquid crystal molecules rise along walls in each direction. Because of this, apparent refractive indices become nearly the same in directions A and B in FIG. 52(b), improving viewing angle characteristics.
For improving the viewing angle characteristics most effectively, liquid crystal molecules in each pixel should be oriented so as to be symmetric with respect to an axis. However, the axisymmetric orientation requires walls, pillars, or the like of a polymer in the middle of the pixels. This leads to problems during practical use, such as the reduction of liquid crystal regions and decreased light transmittance under the application of no voltage. Furthermore, in this case, disclination lines between the liquid crystal domains cannot be controlled, which makes it impossible to eliminate the disclination lines even under the application of voltage. As a result, the display quality is degraded. Alternatively, the decrease in contrast due to difficulties in eliminating disclination lines degrades the display quality.